The present invention relates to flow sensors and pressure sensors, and in particular to such sensors incorporated in an integrated circuit formed on a silicon chip.
Flow, either fluid or gas flow, can be sensed in several different ways. One of these is to provide a suspended wire in the fluid path and measure the amount of heat removed as a function of time. This then produces a signal which can be converted to a flow signal. A second approach is to actually heat up the fluid with a heater and then to measure the heat increase with a sensor element. The advantage of these types of flow sensors is that the flow output is relatively linear with heat removal.
A second approach to flow measurement is to measure the pressure drop across a flow restriction. This is a classic method of measuring flow. In this case, a differential pressure sensor is connected across the restriction and the pressure difference is proportional to the square of the flow per Bernoulli's Theorem. Classically, this has been done in integrated circuit technology with a precision flow channel using a V-groove etching technology in &lt;100&gt; silicon. Channel lengths of several hundred millimeters have been achieved with widths in the 10 to 50 .mu.m range.
Pressure sensors can use a variety of structures. A typical structure is the use of a diaphragm which is exposed to one pressure on one side and a control pressure on the other. The flexing of the diaphragm is then measured and correlated to the pressure. This type of sensor can be incorporated in a silicon integrated circuit using etching techniques, with, as one example, piezo-resistive elements formed on the diaphragm. The resistive elements will vary their resistance in accordance with the stress placed on the diaphragm, allowing measurement of the pressure with an electrical circuit. Alternately, a chip could be bonded to the top of the sensor and capacitive changes could be measured instead of piezoresistive changes.
A combined approach to measuring fluid flow and differential pressure has been implemented using discrete components. This is illustrated in FIG. 1, which conceptually shows the operation. A flow restrictor 10 provides a restricted flow path, while a differential pressure sensor 12 is positioned between pressures P1 and P2. A measurement of the pressure can be done which will take into account both the differential pressure and the fluid flow. The output voltage of an electrical circuit measuring differential pressure would be represented by the following equation: EQU Vout=k*(P.sub.1 -P.sub.2)
where P1 and P2 are the two pressures, k is a constant, and Vout is the measured voltage.
It would be desirable to implement such a structure in a single integrated circuit.